For common three sector cellular (tri-cellular) base station applications, each of three tri-sector antennas usually has 65° 3 dB (half power) azimuth beamwidth (AzBW). Such conventional tri-sector antennas may generate a 65° AzBW with a single column of radiating elements.
Six sector base station cells may be employed to increase system capacity. Antennas with 33°-45° AzBW are the most suitable for six sector applications. A traditional way of narrowing AzBW from 65° to 33°-45° involves employing multiple column arrays of radiating elements arranged on a regular flat reflector with horizontal and vertical spacing to achieve a desired AzBW. For example, for a 45° AzBW antenna, two columns of radiating elements may be arranged about one-half wavelength in horizontal spacing. For a 33° AzBW antenna, it is typical to use three columns of radiating elements arranged about one-half wavelength apart in horizontal spacing.
Each additional column of radiating elements adds to antenna width and feed network complexity. The end result is that for AzBW narrower than 65°, the resultant antenna will include a wider reflector than a regular 65° AzBW antenna, with associated increased weight, wind loading and expense. This is disadvantageous for the space on top of the base station tower at each cell site because operators are sharing the space there.
Lensed antennas have been proposed to modify the beamwidth of an antenna. See, Antenna Engineering Handbook, Fourth Edition, 2007 McGraw-Hill Companies, p. 18-3. The main drawback of this type of antenna is that it requires a large lens with different shapes, which is not acceptable for mounting this type of antenna on the top of the tower with limited space. Additionally, manufacturing this shape of lens may be prohibitively expensive. Another proposed solution is U.S. Pat. No. 4,755,820. In this patent, dielectric loading involves a hemisphere lens, which covers the top half of the antenna. The size of the hemisphere sheet is undesirably large.
Consequently, there is a need to provide a narrower AzBW in a small envelope due to the space limitation on top of the tower, without a large, expensive lens.